Implement replace secondary via the iallocator

This patch implements secondary replace via the iallocator. The new
opcode parameter 'iallocator' behaves like this: if passed, it will
always compute and assign a new secondary, behaving in effect as if the
secondary node has been passed. It conflicts with actually giving the
secondary too.

[Note: not tested with remote_raid1, but the code should behave the
same, we only touch CheckPrereq and we assign a node.]

The patch also adds burnin support for the replace secondary operation;
with this in place, burnin can fully work with auto-assigned nodes.

Reviewed-by: ultrotter